EAGER: Collaborative Research: Developing new laser ablation (U-Th)/(He-Pb) hematite double dating techniques to date ancient oxidation

这个项目的重点是改进地球科学家用来确定矿物赤铁矿形成年龄的方法。赤铁矿是一种广泛存在于岩石和土壤中的矿物，通常是由于地球表面以及其他行星（尤其是火星）的氧化而形成的。赤铁矿在形成时能把铀结合起来。鉴于铀经历放射性衰变，可以对赤铁矿晶体进行铀-铅定年（U-Pb）和铀-钍-氦定年（(U-Th)/He）。研究人员将采用可以在小范围内同时对赤铁矿晶体进行这些测量的方法，即用激光爆破赤铁矿晶体并通过质谱法测量其同位素。赤铁矿的另一个特性是它记录了它结晶时的磁场。这种记录古代磁场的能力使地球科学家能够重建地球大陆过去的位置，也可以用来了解赤铁矿形成的时间。他们将对赤铁矿进行磁测量，包括使用一种称为量子钻石显微镜的仪器在微观尺度上制作磁图的新功能。这项研究将集中研究古代沉积岩中的赤铁矿，这些沉积岩被称为铁地层。在美国，苏必利尔湖地区的铁地层是国内铁生产的主要来源。通过将这些测年方法应用于铁的形成，他们将限制这些单元中赤铁矿的形成时间，其中氧化的时间（即赤铁矿的形成）是有争议的。如果这个项目成功，这些测量赤铁矿年龄的综合方法将使未来的大量研究成为可能。例如，有可能对铁地层中的赤铁矿形成进行更深入的研究，研究深层土壤浸出（也称为红土化）的过程和时间尺度，并使用这些综合方法更详细地确定古代地表暴露和化学变化的时间。该项目将支持一位早期职业科学家的研究，促进加州大学伯克利分校和科罗拉多大学博尔德分校新分析能力的发展，并支持第一代学生的本科生和研究生研究。这些研究人员试图开发同时原位激光烧蚀(U-Th)/He和U-Pb测年赤铁矿，称为LA-(U-Th)/(He- pb)。虽然体积(U-Th)/He和原位（U-Pb）方法已经在赤铁矿上使用过，但耦合激光烧蚀技术从未应用过。该方法可以为评估各种含赤铁矿环境的氧化和风化时间提供有力的工具。研究人员将通过结合古地磁数据的方法研究苏必利尔湖地区铁地层中赤铁矿结晶的时间，这可以提供补充的年代学见解。他们将专注于精心挑选的一组样品，这些样品将使方法开发成为可能，并为铁地层的起源提供新的见解。所有样品将在地质年代学测量之前通过电子背散射衍射和电子显微探针进行表征，以了解样品的化学非均质性和赤铁矿团聚体中晶体的分布。这种表征将允许通过LA-(U-Th)/(He-Pb)靶向单个晶体，并提供与潜在多晶扩散行为相关的背景。本研究有两个主要的样品目标：(1)将用于方法开发的来自铁地层的大型高纯度赤铁矿样品；(2)来自Menominee群的典型铁地层。Menominee组样品将通过LA-(U-Th)/(He-Pb)和古磁学进行分析。古地磁分析将在厘米和微米尺度上进行，以约束赤铁矿相对于褶皱的形成，并通过与Laurentia的表观极移路径进行比较。沉积、构造和近地表风化的区域历史为这些样品中赤铁矿的形成时间提供了可测试的假设。这些材料的成功放射性测年将为LA-(U-Th)/(He-Pb)方法在博物馆级赤铁矿标本以外的自然样品中的应用提供信心。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This project is focused on improving methods that Earth scientists can use to determine the formation age of the mineral hematite. Hematite is a wide-spread mineral in rocks and soils and often forms due to oxidation on Earth’s surface as well as on other planets — notably Mars. Hematite can incorporate uranium when it forms. Given that uranium undergoes radioactive decay it is possible to conduct both uranium-lead dating (U-Pb) and uranium-thorium-helium dating ((U-Th)/He) on hematite crystals. Investigators will apply methods that can make these measurements simultaneously at a small scale on hematite crystals by blasting them with a laser and measuring the isotopes through mass spectrometry. An additional property of hematite is that it records the magnetic field at the time it crystallizes. This ability to record the ancient magnetic field enables Earth scientists to reconstruct the past position of Earth’s continents and can also be used to gain insight into the timing of hematite formation. They will make magnetic measurements of hematite including measurements that use new capabilities to make magnetic maps at the microscopic scale using an instrument called a quantum diamond microscope. The study will focus on hematite within ancient sedimentary rocks known as iron formations. In the United States, iron formations in the Lake Superior region are the major source of domestic iron production. By applying these dating methods to iron formation, they will constrain the timing of hematite formation in these units, where the timing of oxidation (that is, hematite formation) is debated. If this project is successful, these combined methods of measuring the age of hematite will enable a multitude of future studies. For example, it would be possible to pursue more advanced studies on hematite formation within iron formation, to study the processes and timescales of deep soil leaching (also known as laterization), and determine the timing of ancient surface exposure and chemical alteration in far greater detail using these combined methods. The project will support the research of an early-career scientist, advance development of new analytical capabilities at both University of Carlifornia Berkeley and University of Colorado Boulder, and support both undergraduate and graduate student research for first-generation students.These investigators seek to develop simultaneous in situ laser ablation (U-Th)/He and U-Pb dating of hematite, which is termed LA-(U-Th)/(He-Pb). Although bulk (U-Th)/He and in situ (U-Pb) methods have been used previously on hematite, the coupled laser-ablation technique has never been applied. This method can provide a powerful tool for assessing the timing of oxidation and weathering in a wide range of hematite-bearing environments. Researchers will investigate the timing of hematite crystallization in Lake Superior region iron formation through this method development in conjunction with paleomagnetic data, which can provide complementary chronologic insight. They will focus on a carefully selected set of samples that will enable method development and give new insights into the origin of iron formations. All samples will be characterized prior to geochronologic measurements via electron backscatter diffraction and electron microprobe to understand the chemical heterogeneity of the samples and the distribution of crystallites within hematite aggregates. This characterization will permit targeting of individual crystallites through LA-(U-Th)/(He-Pb) and provide context relative to potential polycrystalline diffusion behavior. This study has two main sample targets: (1) large, high-purity hematite samples from iron formation that will be used for method development; (2) typical iron formation from the Menominee Group. Menominee Group samples will be analyzed through both LA-(U-Th)/(He-Pb) and paleomagnetism. Paleomagnetic analyses will be conducted at both the centimeter and micrometer scale to constrain hematite formation relative to folding and through comparison to Laurentia’s apparent polar wander path. The well-constrained regional history of deposition, tectonism, and near-surface weathering provides testable hypotheses for the timing of hematite formation in these samples. Successful radiometric dating of these materials will provide confidence in the utility of the LA-(U-Th)/(He-Pb) method in natural samples beyond museum-quality hematite specimens.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.